For a filter press, it is known to form a filter assembly which consists of a plurality of filter plates arranged alongside one another in pairs, Filter chambers are in each case formed between the filter plates. For this purpose, the filter plates have a central filter area and a thicker sealing rim bordering the central filter area. In the filter press, the filter plates can be moved away from and toward one another, as a result of which the filter chambers can be opened and closed.
Each filter chamber usually has at least one inlet and an outlet. The suspension to be filtered, which is usually designated pulp, flows into the filter chamber via the inlet. The suspension is then filtered by means of a filter cloth, with the solids being held back in the filter chamber by the filter cloth, resulting in the production of what is known as a filter cake. The filtrate freed of the solids is passed out of the filter chamber via the outlet.
In order to reduce the residual moisture in the filter cake and to shorten the filtration times, there are known filter presses in which at least one filter chamber of a filter plate has a diaphragm. Such a filter plate is known as a diaphragm filter plate. In a filter press having diaphragm filter plates, the pressure filtration initially carried out is followed by press filtration, in which the elastic diaphragms of the filter chambers are subjected to a pressure medium, as a result of which they constrict the space in which the filter cake is located. In this way, the filter cake is compressed, and as a result the cavities of the filter cake are reduced in size and as a result residual moisture in these cavities is reduced further.
Such a filter press is described for example in WO 03/095063 A1. In the filter press described therein, a plate assembly is formed which alternately has a filter plate and a diaphragm filter plate. The filter plate has a filter cloth on both sides, said filter cloth being fastened in each case in a cutout in the main body of the filter plate. The diaphragm filter plate has a diaphragm on both sides, said diaphragm being fastened detachably to the main body. With the filter press open, the diaphragm can thus be replaced. With the filter press closed, the diaphragm is held only by the press closing force of the opposite filter plate.
A further filter plate having a detachably fastened diaphragm is known from WO 2006/114235 A1. The replaceable diaphragm is in this case fixed to the main body with the aid of a tongue and groove connection.
In the case of filter plates having a replaceable diaphragm, i.e. a diaphragm that is fastened detachably to the main body, the problem arises that at very high filter pressures of for example greater than 30 bar, the connection between the diaphragm and the main body can lose its sealing during operation of the filter press. Furthermore, it is possible for the diaphragm to detach completely from the main body. In addition, the use of filter plates having replaceable diaphragms is disadvantageous in applications in which it is necessary to sterilize the filter plate after use. This is because, in the case of a filter plate having a replaceable diaphragm, regions which are not sterilizable can occur at the cutouts for receiving the diaphragm.
Furthermore, filter plates are known in which the diaphragm is fastened non-detachably to the main body. In this case, the diaphragm is thus not replaceable. Such filter plates are also known as joint-free. They have the advantage that they are particularly easy to clean.
DE 37 01 862 A1 discloses a filter plate having a diaphragm fastened non-detachably to a main body. The filter plate comprises a supporting wall, a plate frame and what is known as a flexible pressing wall. The pressing wall is formed in one piece with the plate frame and adjoins the latter in the peripheral region of the filter chamber or of the pressure chamber. The rigid plate frame consists of a hard plastics material that is able to withstand the closing pressure of the filter press, while the pressing wall consists of an elastic plastics material. The hard and the elastic plastics materials form a copolymeric phase in a transition region between the plate frame and the pressing wall. In order to produce the plate frame with the pressing wall, a pressing method is proposed in which the pre-manufactured plate frame is introduced into the mold and the elastic plastics material for the pressing wall is added to the mold in the form of granules. In the subsequent pressing operation, the granules are then pressed together and to the plate frame under the action of heat.
Finally, DE 199 43 584 C2 discloses a further filter plate having a non-detachably fastened diaphragm. In this filter plate, a rigid supporting wall is formed in one piece with a rigid plate frame. The diaphragm has an edge strip and is welded by way of the latter to the supporting wall.
A disadvantage with known filter plates having a non-detachably fastened diaphragm is that excess welding material can pass into the region between the main body and the diaphragm. This excess emerges in an uncontrolled manner, such that it disrupts the movement of the diaphragm. As a result, a weak point in the connection between the diaphragm and the main body arises. At high pressures, the diaphragm can be damaged at this weak point. Furthermore, in known filter plates having a non-detachably fastened diaphragm, it is possible for the filter cloth applied to the diaphragm to clog an inlet opening of a drainage bore for the filtrate. This risk of clogging is high in particular at very high filtration and squeezing pressures. However, a filter plate having a non-detachably fastened diaphragm is used precisely under such operating conditions.